ARTICLE

Silicate-replacive high sulfidation massive sulfide orebodiesin a porphyry Cu-Au system: Bor, Serbia

Dina Klimentyeva1 & Thomas Driesner1 & Albrecht von Quadt1 & Trajča Tončić2 & Christoph Heinrich1

Received: 15 April 2020 /Accepted: 20 October 2020# The Author(s) 2020

AbstractThe Cu-Au deposit of Bor (Serbia) represents a continuum of mineralization styles, from porphyry-style ore occurring in quartz-magnetite-chalcopyrite veins and chalcopyrite disseminations to high-sulfidation epithermal Cu-Au ores in pyrite-chalcopyriteand anhydrite-sulfide veins. Decisive for the great economic importance of Bor is the presence of exceptionally rich high-sulfidation massive sulfide orebodies, composed of pyrite + covellite + chalcocite/digenite and minor anhydrite and enargite.They form irregular bodies measuring 0.5–10 million tons of ore grading up to 7% Cu, hosted by andesites and surrounded byintense argillic alteration. This study focuses on a small but rich underground orebody mined out recently, where limited drillcoreis preserved for quantitative geochemical study. This paper documents the vein relationships within the deep porphyry-styleorebody of Borska Reka, the transitional porphyry-epithermal veins, and the overlying and laterally surrounding epithermalmassive sulfides of the Bor deposit. Geological observations indicate that the formation of massive sulfide orebodies concludesthe ore formation.Mass balance calculations, recast into geologically realistic bulk fluid-rock reactions, confirm textural evidencethat near-isovolumetric replacement of andesite host rocks is the dominant formation mechanism of massive sulfide orebodies atBor, whereby all lithophile elements including Si are dissolved and only Ti stays relatively immobile. While net volume changesfor individual mineralization styles within the massive sulfide orebody vary from − 16% volume loss to + 127% volume gain,overall volume change for the whole massive sulfide orebody was probably slightly negative. Brecciation is important only asmeans of creating channelways for reactive fluid that turns the andesite protolith into massive sulfide, whereas net breccia infilloccurred only locally.

Keywords Porphyry Cu-Au .Massive sulfide .Mass balance

Introduction

Porphyry Cu-Au systems (Hedenquist and Lowenstern 1994;Sillitoe 2010) are magmatic-hydrothermal ore systems thatcommonly include two spatially associated and geneticallylinked styles of Cu-Au ore. Porphyry-style mineralization

holds the bulk of the economic ore in large-tonnage but rela-tively low-grade orebodies dominated by chalcopyrite ± born-ite in and around stockwork veinlets. Epithermal mineraliza-tion typically overlies or is adjacent to the porphyry ores andpostdates them and mostly occurs in swarms of discrete veinsor more diffuse fracture zones containing pyrite with high-sulfidation Fe-Cu-As-Au-S assemblages. Examples of this as-sociation include Butte, Montana (Meyer et al. 1968; Rusket al. 2008), Far Southeast-Lepanto in the Philippines(Mancano and Campbell 1995; Arribas et al. 1995b;Hedenquist et al. 1998), La Famatina in Argentina (Losada-Calderón and McPhail 1996; Pudack et al. 2009), Elatsite –Chelopech in Bulgaria (Chambefort and Moritz 2014), OyuTolgoi inMongolia (Crane et al. 2012), and Valeriano in Chile(Sillitoe et al. 2016). The high-sulfidation assemblages com-prise abundant pyrite with enargite + covellite ± digenite ±native sulfur. These are typically associated with advancedargillic alteration with residual quartz + alunite ± kaolinite ±

Editorial handling: B. Lehmann

Supplementary Information The online version containssupplementary material available at https://doi.org/10.1007/s00126-020-01023-2.

* Dina KlimentyevaDina.klimentyeva@erdw.ethz.ch

1 Institute of Geochemistry and Petrology, ETH Zürich,8092 Zürich, Switzerland

2 Zijin Bor Copper D.O.O., Bor 19210, Serbia

Mineralium Depositahttps://doi.org/10.1007/s00126-020-01023-2

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anhydrite, giving rise to the alternative definition of the de-posits as kaolinite-alunite-type epithermal deposits (Healdet al. 1987; Berger and Henley 1989). Silica-rich alteration(commonly named “vuggy quartz”) is typically generated inan early stage of the paragenetic evolution, probably by mag-matic vapor in a sub-fumarolic environment (Stoffregen 1987;Arribas et al. 1995a; Heinrich et al. 2004). By contrast, latersulfide mineralization into vuggy quartz is dominated by low-salinity aqueous fluids (Mancano and Campbell 1995; Arribaset al. 1995a, b) that may have a deeper magmatic originfollowed by cooling and contraction of a single-phase inter-mediate-density fluid (Arribas 1995; Hedenquist et al. 1998)or magmatic vapor that had condensed some brine in the por-phyry environment at depth (Henley and McNabb 1978;Heinrich 2005). Thereby the low-salinity magmatic fluid be-comes miscible with meteoric water (Sillitoe 1983) or seawa-ter (Gruen et al. 2014) but also allows re-mixing withentrained magmatic brine (Hedenquist et al. 1994; Wanget al. 1999). An entirely different idea calls on direct conden-sation of an As-rich sulfide melt from high-temperature mag-matic gas (Mavrogenes et al. 2010).

Many high-sulfidation Cu-Au ores are silica-rich and, dueto their environmentally detrimental content of enargite andother As or Sb-rich sulfosalts, are rarely mined for Cu andmainly gain major economic significance as easily leachableAu deposits after pervasive supergene oxidation, e.g.,Yanacocha (Longo et al. 2010), Pierina (Rainbow 2009),and Zijinshan (Chen et al. 2019; Pan et al. 2019). However,a few porphyry systems contain massive high-sulfidationorebodies that are of such high copper grade that they becameworld-class base metal deposits in their own right and werehistorically mined even before the lower-grade porphyry oresbecame exploitable. The most important example is Butte(Montana) where a swarm of major open fractures was filledby high-sulfidation sulfide minerals forming the Main StageVeins (Weed 1912; Meyer et al. 1968). At Cerro de Pasco,polymetallic Zn-Pb-Cu ores with massive pyrite are formed atthe contact between a diatreme and the adjacent limestone bycarbonate replacement, which caused the neutralization oflow-pH fluids (Baumgartner et al. 2008; Rottier et al. 2016).More enigmatic deposits are dominated by massive sulfideorebodies of irregular, tabular, or pipe-like shape lacking ob-vious evidence of open space creation or reaction with readilydissolvable carbonate host rocks. These include Lepanto in thePhilippines, Frieda River in Papua New Guinea, Recsk inHungary, Chelopech and Radka in Bulgaria, and Bor inSerbia (Sillitoe 1983; Popov 2001; Chambefort and Moritz2014). Bor had great historic importance as one of the largestand highest-grade copper deposits of Europe (Lazarević 1909;Mempel 1937; Schumacher 1954; Janković et al. 1980;Janković 1990; Janković et al. 2002; Koželj 2002;Jelenković et al. 2016; Knaak et al. 2016). The deposit is ofrenewed interest thanks to the recent discovery of a similar

high-grade orebody at Cukaru Peki (Banješević et al. 2014;Jelenković et al. 2016; Koželj et al. 2016; Jakubec et al. 2018).The origin of these massive sulfide deposits has been debated,initially being interpreted as submarine-exhalative volcanic-hosted massive sulfide deposits due to their association withpartly submarine volcanics and locally layered pyrite textures(Drovenik 1953; Drovenik et al. 1975; Bogdanov 1984).Today, they are accepted to be variants of high-sulfidationepithermal deposits due to their clearly epigenetic nature andtheir association with porphyry ore systems (Sillitoe 1980;Kouzmanov et al. 2009; Chambefort and Moritz 2014).However, the nature and origin of these sharply boundedorebodies, typically measuring tens of meters in all directionsand Cu grades reaching 10%, remain poorly described andunderstood despite their economic attractiveness. Althoughbrecciation and open-space filling are observed locally(Chambefort 2005), we show here that one of the largest andrichest deposits of this type is formed by largely texture-preserving replacement of volcanic rocks, as suggested al-ready by Sillitoe (1980).

The copper-gold deposit of Bor in Serbia has recently be-come partly accessible to research in the course of renewedexploration, based on a cooperation between international in-vestors and the Serbian State as main owner (Jamasmie 2018).This paper is a new geological description of related ore typesin the deposit, which includes a deep porphyry-style veinstockwork, as the largest remaining resource, and partly pre-served higher levels accessible in exploration adits, includinga recently mined massive sulfide orebody. The main focus ofthe paper is to combine textural evidence with mass balancedata based on bulk rock analyses, to test whether the massivesulfide orebodies indeed formed by nearly isovolumetric re-placement of volcanic rocks. These results will later be used tomodel possible fluid evolution paths that could generate suchunusual, easily missed but high-value orebodies.

Geological background

The Apuseni-Banat-Timok-Srednogorie belt (ABTS, Fig. 1inset) is a magmatic arc that was active during the LateCretaceous (92–67 Ma) and formed due to the subduction ofthe Vardar Ocean (part of the Neo-Tethys) beneath the com-plex southern continental margin of the Eurasian plate. ThisAndean-type arc was preceded by earlier nappe emplacementand later overprinted by the effects of continental collisionwith the Adriatic microplate during Tertiary times (Kolbet al. 2012; Gallhofer et al. 2015). In the period from 92 to67 Ma, arc magmatism became younger away from theEuropean continental margin, towards the west in Serbia andtowards the south in Bulgaria (Gallhofer et al. 2015), which isinterpreted to reflect progressive roll-back of the subductedslab (Von Quadt et al. 2002; Von Quadt et al. 2005; Kolb

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et al. 2012). After its emplacement, the arc was bent aroundthe European foreland as a result of syn- to post-collisionaldeformation (Schmid et al. 2008; Knaak et al. 2016), includ-ing more than 100 km of accumulated dextral movementalong the Timok-Cerna fault system (CF – TF in Fig. 1, inset;Matenco and Radivojević 2012). Regional geology and geo-chronology of the Bor metallogenic zone and the Timok mag-matic complex (TMC), the largest area of preserved volcanicrocks in the ABTS belt (Fig. 1), have been summarized byAntula (1909), Schumacher (1954), Janković (1977), VonQuadt et al. (2002), Banješević (2010), Gallhofer et al.

(2015), Jelenković et al. (2016), and Knaak et al. (2016).Neoproterozoic and Paleozoic metamorphic rocks constitutethe basement of the TMC, which is deformed and overlain byJurassic to Early Cretaceous limestones and sandstones.Albian glauconitic sandstone rests unconformably on Aptianlimestones, overlain by the volcanic rocks that range in com-position from basaltic andesites to andesites (Banješević2010).

The volcanic activity that formed the TMC took place dur-ing the Turonian stage of the Late Cretaceous epoch, with anestimated onset around 87 Ma (Kolb 2011; Gallhofer et al.

Fig. 1 Southeastern area of the Timok Magmatic Complex and thelocation of the Timok Magmatic Complex within the Apuseni-Banat-Timok-Srednogorie belt (inset). Modified after Mioč (1983), Đorđevićand Banješević (1997), Knaak et al. (2016), and Pittuck and McGurk

(2016). CF, Cerna fault system; TF, Timok fault system; MF, Maritsafault system. Line A – A’ marks the position of the depositsection in Fig. 2

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2015). It is sub-divided into three phases, two of which aresignificant for the Bor deposit geology (Fig. 1). Phase I, whichhosts the deposits of Bor and Veliki Krivelj, is represented byconspicuously porphyritic-textured high potassium andesites,dominantly as lava flows but also as lava domes and somepyroclastic breccias, exposed mainly in the eastern part of theTMC. Euhedral amphibole phenocrysts are prominent andcommonly reach a centimeter in size; zoned plagioclase with40–50% anorthite component (Janković et al. 2002) is similarto amphibole in terms of abundance at about 25–35%, whilebiotite, magnetite, and quartz are subordinate. Occasional py-roxene represents a minor component of the phenocryst as-semblage. The matrix contains fine-grained plagioclase, bio-tite, and quartz and amounts to 40–50% of the rock(Banješević 2010; Antonijević and Mijatović 2014).

Phase II, which comprises most of the area of the Timokcomplex, is represented by more matrix-rich (> 50%)pyroxene-phyric basaltic andesites, which locally show evi-dence of submarine extrusion (Banješević 2010). Augiteclinopyroxene and plagioclase with 42–93% anorthite contentare prevalent among the phenocrysts and are generally smallerthan the phenocrysts of the phase I andesite; enstatite andtschermakitic amphibole locally are subordinate phenocrystcomponents (Banješević 2010). These rocks are ofSantonian to Lower Campanian age based on fossil evidencefrom the interbedded marine sedimentary rocks (Banješevićet al. 2006).

The concluding phase III of magmatic activity is manifest-ed in latites that are outcropping in the southwestern part of theTMC, dominantly as small-scale dikes and subvolcanic intru-sions, rarely as lava flows, and are composed of plagioclaseand K-feldspar phenocrysts set in a fine-grained tohypocrystalline matrix (Banješević 2010). The age of the latiteis defined as Campanian to Lower Maastrichtian (Karamataet al. 1997).

The Bor metallogenic zone is located near the eastern mar-gin of the TMC (Fig. 1) extending from Serbia into Romania(Grubić 1974; Janković 1977) and contains the porphyry cop-per deposits of Majdanpek and Veliki Krivelj, the porphyry—high-sulfidation epithermal deposits of Bor and recently dis-covered Cukaru Peki (Antula 1909; Schumacher 1954;Janković 1997; Karamata et al. 1997; Berza et al. 1998;Jelenković et al. 2016). Total Cu endowment of the Bormetallogenic zone is estimated at 20 million metric tons(Mt) of Cu and 1000 t of Au (Jelenković et al. 2016); mea-sured and indicated resources of the Bor deposit (dominatedby Borska Reka and Tilva Ros) amount to 5.8 Mt Cu at agrade of 0.3% and 260 t Au (Tončić 2018), while the totalendowment of Bor group of deposits is estimated at 6.72 Mtand a grade of 0.84% (Baker 2019).

The Bor deposit is hosted by hornblende andesite andpyroxene-hornblende andesite; these are referred to as phaseI and phase II andesites, respectively (Đorđević and

Banješević 1997; Đorđević 2005; Banješević 2010). The de-posit is truncated at the base and eastern side by the Bor faultand underlying Bor conglomerate, a former late- to post-depositional graben structure that was subsequently re-activated by overthrusting of the package of mineralized vol-canic rocks towards the east (Janković et al. 2002; Đorđević2005). All deposits of the Bor metallogenic zone (Fig. 1),including the Veliki Krivelj porphyry and the new CukaruPeki, are hosted by andesites of phase I and are overlain byepiclastic layers of andesitic and dacitic composition of phaseII that are intercalated with pelites (Fig. 2; Koželj 2002;Jelenković et al. 2016).

Age and duration of mineralization at Bor are poorlyconstrained between ~ 84.5 and ~ 82 Ma (Fig. 3). Ages ofphase I andesites in the vicinity of Veliki Krivelj wereestablished by ID-TIMS of zircons at 86.29 ± 0.32 Ma and84.17 ± 0.86 Ma, overlapping with an approximate LA-ICPMS age of 84.28 ± 0.86 Ma for a sample of dacite in closeproximity to the Bor mine (Kolb 2011). A trachyte dike thatcuts phase II andesites 5 km south of Bor yielded an ID-TIMSage of 82.35 ± 0.35 Ma as the only well-dated post-mineralintrusion (Kolb 2011). 40Ar-39Ar plateau ages of igneoushornblende from phase I andesite in the vicinity of Bor arereported as 83.4 ± 1.7Ma and 84.6 ± 1.1Ma (Lips et al. 2004).Alteration minerals at Bor were dated by 40Ar-39Ar and K-Armethods and overlap within the analytical uncertainty with themagmatic ages of the dacite, with 84.8 ± 1.9 Ma, 86.9 ±1.1 Ma and 85.9 ± 1.7 Ma for white mica from the BorskaReka porphyry ore and 84.6 ± 1.2 Ma for alunite from theadvanced argillic alteration zone in the Bor mine (Lips et al.2004; Lerouge et al. 2005). Re-Os dating of molybdenite fromBor yielded significantly older ages of 86.24 ± 0.5 Ma and85.94 ± 0.4 Ma (Zimmerman et al. 2008), similar to the olderID-TIMS age at Veliki Krivelj. Magmatic and hydrothermalactivity at Bor probably occurred around 84–85 Ma andwas related to late Phase I magmatism, as indicated by40Ar-39Ar and K-Ar ages of alunite and white mica and40Ar-39Ar age of hornblende of phase I andesite fromthe Bor mine and U-Pb zircon age of the dacite in thevicinity of Bor mine. We suspect that Re-Os ages ofmolybdenite are probably overestimated.

The orebodies at Bor are hosted entirely by phase I andes-ites (Figs. 1 and 2). The least-altered andesites from the pe-riphery of Borska Reka display the typical porphyritic texturedefined by euhedral amphibole, plagioclase, and subhedralbiotite phenocrysts, partially altered to chlorite and sericitewith minor pyrite and chalcopyrite.

Diorite porphyry dikes, 50–150 m wide, were logged at thedeep levels of Borska Reka (Janković et al. 1980), but theirtrue dip direction is uncertain because no core is preserved.Shallower, small-scale porphyry dikes are present in the cen-tral part of Borska Reka (T. Toncić, pers. commun.), and asingle 1.5-m core interval was tentatively logged as a diorite

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dike during our field campaign. Largely texture-destructivealteration precludes reliable mapping of intrusive contacts,

but an indication of crowded equigranular texture and thelarger abundance of zircons compared to immediately

Fig. 2 Cross-section through the Bor deposit, showing the apparentcontinuum between porphyry-style ores at Borska Reka and epithermalore types, including the former Tilva Ros massive sulfide body in the openpit, combining personal observations and unpublished mine sections. aColored grade contours delineate the projected mineralization based oncurrent assay data for the Borska Reka porphyry (solid lines), andapproximate position of mineralization and alteration zones for Tilva Rosand Brezanik orebodies based on historic assay data and interpolation(dashed). Position, thickness, and orientation of porphyry dikes areuncertain projections from mine data. Alteration types within the phase I

andesites are confirmed in the central region by partly preserved drillcore(vertical lines) and near T orebody, or interpolated and projected from othermine sections elsewhere. b Cross-section through T massive sulfideorebody, showing the zonation of the mineral assemblages, from bornite+ chalcopyrite + pyrite in the bottom part of the orebody to predominantlycovellite with pyrite in the central part, and chalcocite + pyrite in the rim.Abbreviations: bn, bornite; ccp, chalcopyrite; cct, chalcocite; cv, covellite;hm, hematite; mag, magnetite; py, pyrite. After Starostin (1970), Jankovićet al. (2002), and Jelenković et al. (2016), with additions based on Borminegeologic documentation

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neighboring andesite are evidence in favor of a subvolcanicintrusive origin (sample BGM1–64.8).

The Bor conglomerates (Fig. 4) outcrop to the east and to thesouth of Bor and are observed in the Bor mine as footwall unit

below the Bor fault (Fig. 2). The Bor conglomerate containsrounded clasts of basement gneiss, andesite with kaolinite al-teration, and limestone and marls of Campanian toMaastrichtian age (~ 72–66 Ma) based on planktonic

Fig. 4 Main lithologies at Bor. a Bor conglomerates from − 155 minelevel. b Pelite, base of phase II andesite overlying phase I andesite, B67-35.8. c Phase I andesite with weak chlorite-sericite alteration, anhydrite-

gypsum vein; BGM12-86.3. Sample labels indicate drill hole numbers(i.e., BGM12) and depth (i.e., 86.3 m). See Fig. 2 for the location of thedrill holes

Fig. 3 Geochronology of the Bor deposit, with likely age brackets encompassing all ore types indicated by the gray bar (Lips et al. 2004; Lerouge et al.2005; Von Quadt et al. 2007; Zimmerman et al. 2008; Kolb 2011)

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foraminifera (Janković et al. 2002; Ljubović-Obradović et al.2011). To the south of Bor, conglomerates made up of miner-alized clasts constitute the deposit of Novo Okno, with clasts ofcovellite + pyrite in the lower part, chalcopyrite + bornite in themiddle, and pyrite + chalcopyrite on top and at the periphery ofthe deposit (Antonijević 2011), representing an inverted se-quence observed within the massive sulfide bodies at Bor andindicating exposure and erosion of the deposit by LateCretaceous time. Outside the mine area, the conglomeratesare overlain by Maastrichtian sandstone and claystonewith Gansserina Gansseri, which defines their youngestage as Maastrichtian (Ljubović-Obradović et al. 2011),indicating exposure of the Bor deposit some 10–15 Maafter its formation.

Geology of the Bor deposit

A range of mineralization styles is present at Bor, varyingfrom the deep porphyry-style stockwork orebody of BorskaReka to the shallow epithermal mineralization of the TilvaRos orebody, the former center of a large open pit mine thatwas closed in 1993 and is inaccessible today (Fig. 2a). Most ofthe massive sulfide orebodies including Tilva Ros and thesmaller L, T, T1, T2, and P2A massive sulfide bodies arelocated in the upper and southeastern margin of the deposit(Fig. 2; Janković et al. 2002; Koželj 2002). Remnants of themassive sulfide orebody T, high-sulfidation veins in the shal-low Brezanik underground mine, and parts of the transitionabove Borska Reka are accessible in development tunnels,and together with minor preserved core from the deeper partof Borska Reka are the basis of this study. In the followingsub-sections, four main styles of mineralization with charac-teristic vein types (Fig. 5) and macroscopic alteration andreplacement relationships (Fig. 6) are described; microscopicevidence is presented in support of the suggested mineraliza-tion sequence (Fig. 7).

Weakly altered phase I andesites are intersected by shallowdrill holes south at the level of the Tilva Ros epithermalorebody (Figs. 4c and 6a) and are characterized by partlypreserved igneous amphibole, biotite, and relict magnetite(Fig. 7a). Plagioclase displays twinning and oscillatory zoningin the rim, with An-rich cores replaced by kaolinite, sericite,and anhydrite. Magnetite preserves the original grain shape,but shows dissolution pits, and in rare cases is partly replacedby hematite. The matrix contains unaltered plagioclase lathstogether with quartz, some fine-grained magnetite and chalco-pyrite, and chlorite and accessory rutile. No evidence for strat-igraphic orientation was observed within the massive andes-ites, but the overlying transitional shales to phase II andesitessuggest subhorizontal disposition.

Porphyry-style stockwork ore with chlorite-sericitealteration is contained in magnetite-chalcopyrite-quartz veins

and chalcopyrite disseminations in the deep part of BorskaReka, where alteration of the andesite is dominated by chloriteand sericite which probably overprinted potassic alteration insome places, as indicated by relictic magnetite although nohydrothermal biotite or K-feldspar is preserved (Figs. 5a and7b). The upright cylindrical orebody is truncated by the barrenBor fault; the Bor conglomerates intersected by deep drillholes do not contain any amount of copper sulfides. In theupper parts of Borska Reka, barren quartz veins and AB-type quartz-chalcopyrite-pyrite ± hematite veins, cross-cutby tiny pyrite ± hematite stringers, become the main veintypes, in wallrocks altered to quartz and sericite with variableoverprint by anhydrite and kaolinite (Fig. 5b). In the chlorite-sericite alteration assemblage, most of the amphibole pheno-crysts are replaced by chlorite, with significant amount ofpyrite, sometimes with inclusions of co-entrapped pyrrhotite+ chalcopyrite, chalcopyrite, and magnetite (Fig. 7b). Somerelict quartz phenocrysts are preserved. Rutile precipitatesalong the former cleavage planes of amphibole and magnetite,and rims of rutile are formed around magnetite; the fine-grained matrix consists of quartz and sericite. Magnetite-chalcopyrite-quartz veins are composed of euhedral magne-tite, altered at the rims to hematite, and anhedral chalcopyrite.Outer parts of pyrite overgrow and include both magnetite andchalcopyrite. Cores of the pyrite host inclusions of pyrrhotiteintergrown with chalcopyrite, followed by chalcopyrite-onlyinclusions towards the rim. No intersection relationships wereobserved between magnetite-chalcopyrite veins and othervein types, but at shallower levels tiny magnetite veinscross-cut A-type quartz veins and are cross-cut by pyrite ±chalcopyrite veins (Fig. 5c).

Chalcopyrite is present in tiny amounts as disseminationsand as inclusions in pyrite, and in rare cases, chalcocite ±covellite overgrows fractured pyrite together with hematiteand anhydrite. The mineral name chalcocite is used in thispaper as the overarching term for chalcocite group mineralsincluding digenite. Where coexisting with covellite + pyrite inthe ore matrix or late veins, chalcocite is usually gray with agreenish tint and somewhat higher reflectance than digenite inpolished section; chalcocite predominates at Bor, except forsmall digenite inclusions in pyrite (commonly with bornite)which have the characteristic blue-gray color and reflectance.Quartz-sericite alteration is associated with somewhat lowerCu grade (0.3–0.7 wt% Cu) than the best porphyry ore deeperin the section (0.7–1wt%Cu), with onlyminor chalcopyrite inthe lower part and small anhydrite-chalcocite-hematite vein-lets in the upper part of the interval, the latter presumably aweak expression of the shallow epithermal vein style resultingfrom re-opening of pyrite ± hematite stringer veins. Very localmonomictic clast-supported breccia is associated with the up-per boundary of the quartz-sericite alteration zone, with clastscomposed of quartz, pyrite with rare pyrrhotite inclusions,kaolinite, and rutile ± hematite, set in the matrix comprising

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quartz, anhydrite, pyrite, rutile ± digenite, covellite, andcolusite.

Low-grade high-sulfidation Cu ore with kaolinite-anhydrite alteration is transitional between the Borska Rekaporphyry style and the Tilva Ros massive sulfides. This volu-metrically dominant epithermal alteration in Fig. 2 corre-sponds to variably siliceous advanced argillic alteration in-cluding minor vuggy quartz. Compared with other epithermaldeposits, it stands out due to the high abundance of anhydrite

and only local occurrence minor alunite. It is characterized by0.5–1-cm-thick veins with pyrite ± chalcopyrite, and 1–2-cm-thick anhydrite-sulfide veins with pyrite, covellite, enargite,and chalcocite, in wallrock alteration of kaolinite, anhydrite,and diaspore at the upper levels of the deposit (Figs. 5e and 7),with relict patches of chlorite-sericite alteration at the periph-ery. Both pyrite-chalcopyrite and anhydrite-sulfide veinscross-cut quartz A-type veins and quartz-pyrite AB-typeveins. Thickness and abundance of anhydrite-sulfide veins

Fig. 5 Vein and alteration styles at Bor; for location of the drill hole seeFig. 2. aMagnetite-chalcopyrite-quartz vein in the wallrock with chlorite-sericite alteration; BGM7-316. b Chalcopyrite-pyrite veins cross-cuttingquartz-pyrite AB-type veins, upper zone of the chlorite-sericite alterationdomain; BGM3-301. cQuartz A-type veins cut bymagnetite veinlets andpyrite microcracks, covellite disseminations, kaolinite-anhydrite alter-ation; suspected porphyry; before BGM1-64.8. d Pervasive silicificationabove chlorite-sericite alteration zone with some residual chlorite (green),pyrite-hematite vein; BGM3-212. e Anhydrite-sulfide vein with solidsulfur cutting wallrock with kaolinite-anhydrite alteration; BGM6-63.5.

f Quartz AB-type vein with pyrite-chalcopyrite centerline cross-cut bypyrite-chalcopyrite veinlets and by a late anhydrite-pyrite vein, covellitedisseminations; BGM1-6.5. g Quartz-covellite-enargite vein from the pe-riphery of the T orebody in a matrix with disseminated pyrite, covelliteand enargite; BC18-40.1. h High-grade massive sulfide ore from thecentral part of the T orebody: deformed quartz vein, A or AB-type withpyrite, chalcocite and minor anhydrite in the matrix of massive sulfide;BB4-48. Abbreviations: anh, anhydrite; ccp, chalcopyrite; cct, chalcocite;cv, covellite; eng, enargite; hem, hematite; mag, magnetite; py, pyrite; qz,quartz; s, native sulfur

Miner Deposita

increase upwards; in the upper tunnels just below Tilva Ros, a20-cm-thick anhydrite-pyrite-covellite vein was observed.Generally, anhydrite-sulfide veins cross-cut pyrite ± chalco-pyrite veins (Fig. 5f), but the reverse cross-cutting relation-ships can occasionally be found, white anhydrite is the mostimportant constituent in the veins and is prominent as an al-teration mineral in this interval. Pyrite within anhydrite-sulfide veins is fractured and overgrown by spectacular bladesof covellite, anhydrite, and chalcocite as well as minorenargite. Etching of the pyrite from high-sulfidationepithermal veins revealed at least two generations of pyrite(Fig. 8d). Apart from anhydrite-sulfide veins, copper is dis-seminated as covellite and as chalcopyrite inclusions in pyrite.Native sulfur was found in two samples, in the central part ofanhydrite-sulfide veins (Fig. 5e). In contrast to the well-preserved phenocryst shapes in the chlorite-sericite alterationassemblage, former grain boundaries of the phenocrysts arebarely discernible within the kaolinite-anhydrite alteration do-main (Fig. 5e, f).

High-grade massive sulfide ores exemplified by the Torebody (Fig. 2b) are distinguished by the highest ore grade,

with 50–90% of the host rock replaced by sulfide minerals,namely pyrite, covellite, chalcocite, and enargite in decreasingabundance (Fig. 6). The massive sulfide T orebody has anequidimensional round shape with a diameter of ~ 50 m, isnot delineated by any conspicuous extensional structures, andshows no major breccia infill textures, but seems to be formedby wholesale hydrothermal replacement of the host andesite.A detailed description of the representative sample suite fromthis orebody and its vicinity is presented in Digital AppendixA. Background alteration outside of the T orebody is similar tothe kaolinite-anhydrite alteration elsewhere; inside the Torebody, the alteration is advanced argillic, with increasedabundance of diaspore and decreased abundance of anhydrite.

In the most sulfide-rich samples from the high-grade zoneof the T orebody quartz is present only as small, 100-μmgrains and fragments in the rock that is dominantly composedof chalcocite and pyrite (Fig. 6g). Relict quartz A or AB-typeveins observed within the massive sulfide orebodies are de-formed and fractured, the fractures being filled with chalcocite(Fig. 5h). In the central high-grade part of the T orebody, smallveinlets of covellite cut through massive sulfide with about

Fig. 6 Replacive textures advancing from andesite protolith to massivesulfide in and around T orebody, in representative samples described inthe Digital Appendix A and used for geochemical mass balance analysis;for location of the drill hole see Fig. 2b. aWeakly altered phase I andesiteprotolith with euhedral amphibole and plagioclase phenocrysts; B67-258.b Monomictic breccia at the base of T orebody, with two types of clastscemented by pyrite-richmatrix: partly sulfidized andesite clasts (red lines)with phyllic-argillic alteration and hydrothermal vein clasts of earlierhydrothermal quartz-sulfide AB-type veins; BB4-2.5. c Silicified ad-vanced argillic breccia with relict former amphibole phenocrysts replaced

by pyrite; BB1-b2. dAdvanced argillic alteration, pyrite and chalcopyriteveins; BA2-85.5. e Vuggy residual quartz with pyrite and kaolinite in thevugs, minor rutile; BB4-55.8. f High-grade massive sulfide ore withquartz from the central part of the orebody: pyrite showing relict shapesof replaced amphibole phenocrysts in a matrix of covellite, chalcocite,minor enargite, and residual quartz, with a late covellite veinlet cross-cutting pyrite aggregates; BB3-23. g Completely recrystallized massivesulfide ore with euhedral pyrite grains and aggregates with inclusions ofchalcocite and digenite, growing in a matrix of chalcocite and covellite;BB4-48

Miner Deposita

30–50% residual quartz, former phenocrysts of andesiteprotolith replaced by pyrite with inclusions of copper sulfidesand overgrown by fine-grained pyrite rim (Fig. 6f). The relicttexture of magnetite phenocrysts replaced by pyrite and rutilecan be seen in the sample from the T orebody’s periphery (Fig.7f). Quartz-enargite-chalcocite-covellite veinlets from theedge of the massive sulfide orebody display deformation andovergrowth of enargite by covellite and chalcocite; anhydrite-covellite veins exist in the proximity to the massive sulfidedeposits, and quartz-covellite-enargite veins with minor anhy-drite are found in the outer margins of the T orebody (Fig. 5g).One- to 2-cm-thick veins of chalcopyrite + pyrite occur at theperiphery and below the massive sulfide T and T1 orebodies.

The massive sulfide T orebody is capped by a zone ofvuggy residual quartz and framed by local breccias, pervasivesilicification patches, and small-scale, steep-dipping faultzones, but these faults do not seem to delineate the roundshape of the body (Fig. 3b). The contact breccias are matrix-supported, monomictic, with variably altered clasts includingpervasively silicified clasts, as well as clasts of andesite withphyllic-argillic alteration and bornite, chalcopyrite, andenargite (Fig. 6b); clasts of vuggy quartz occur locally. Itspores constitute around 30% by volume and are partlyfilled with pyrite, covellite, free-standing quartz crystalsand occasional barite, alunite, anhydrite, and kaolinite.In the clasts with bornite and chalcopyrite, rare euhedral

Fig. 7 Mineral paragenesis based on microscopic observations. a Least-altered andesite with residual amphibole, plagioclase, and igneous mag-netite; B67-258. b Former amphibole phenocryst altered to chlorite-sericite, disseminated chalcopyrite; BGM7-316. c Anhydrite-sulfide veinwith two-stage pyrite, etched with concentrated HNO3; B8 from the TilvaRos decline at − 130 m. d Anhydrite-sulfide vein with fragmented pyriteovergrown by anhydrite and covellite, rutile, and chalcopyrite as inclu-sions in pyrite; BGM1-100. e SEM phase map of texture-destructivesiliceous kaolinite-anhydrite alteration with disseminated diaspore, covel-lite and rutile; BGM1-195. f Magnetite replaced completely by pyrite +

rutile along former parting planes, and a pyrite-chalcopyrite vein in theadvanced argillic alteration zone at the edge of the T orebody; BA2-85.5.g Massive sulfide high-grade ore comprised of euhedral fractured pyriteset in chalcocite matrix, with rutile closely associated with residualquartz; BB4-48. h Anhydrite-sulfide veins from the kaolinite-anhydritealteration domain, with enargite + chalcocite overgrown by covellite andfine-grained pyrite; BGM12-113.3. i Residual A or AB-type quartz veinin the massive sulfide, with enargite fractured and overgrown by covel-lite, minor anhydrite; BC18-40.1

Miner Deposita

enargite crystals are fractured and overgrown by borniteand chalcopyrite. Apart from the pores, vuggy quartzalso exhibits ghost phenocrysts replaced by pyrite andrutile (Fig. 6e).

Observation of partially preserved drill core suggests thefollowingmineral zonation within the T orebody (Figs. 2b and6). Bornite is observed only within clasts in a breccia in thebottom part of the T orebody, and also the lower part of theneighboring T1 orebody contains bornite + pyrite in apprecia-ble amounts. The center of the T orebody is massive pyritewith covellite > chalcocite. Edges of the orebody are charac-terized by lesser abundance of covellite, with chalcocite beingthe dominant copper sulfide phase. A decrease in coppergrades at the edges of the T orebody coincides with the ele-vated content of pyrite. Veinlets of enargite, as well as enargitein disseminations, are observed more frequently at the edgesof the orebody, while pyrite-chalcopyrite mineralization ispresent at the outer margins of the T orebody. Digenite ispresent as inclusions in the outer rims of pyrite in the high-grade, chalcocite-dominated massive sulfide.

Figure 8 summarizes the temporal overprinting relationsobserved at Bor at the sample to thin section scale, but despitesearching, we found no large-scale structural timelines (e.g.dikes truncating veins) to exclude the possibility that this al-teration evolution progressed essentially together in a singlezoned hydrothermal system. As the earliest hydrothermalproduct, sinuous granular A-type quartz veins formed, practi-cally free of sulfides and cross-cut by all the other vein types.Magnetite-chalcopyrite-quartz veins were probably formeddue to the re-opening of the early granular A-type quartz veins(Fig. 5a) and represent the earliest mineralization contempo-raneous with potassic alteration, later overprinted by chlorite-sericite. Chlorite-sericite alteration is overprinted first byquartz-sericite and later by kaolinite-anhydrite alteration,which is well expressed in the shallow part of Borska Rekazone between Borska Reka porphyry and Tilva Rosepithermal orebody and in themarginal parts of the T orebody,

where local zones of vuggy quartz occur (Fig. 6d, e). Pyrite-chalcopyrite veinlets post-date quartz veins, form centerlinesof AB-type veins and were probably re-opened during thelater ore precipitation event, when anhydrite-sulfide veinsformed. Although no clear cross-cutting relationships are ob-served between chalcopyrite-pyrite veins and the massive sul-fide mineralization, the spatial association of breccias to thelocal silicification zones overprinted later by pyrite-chalcocite(Fig. 6c) and the presence of quartz-pyrite AB-type veinclasts, as well as clasts with bornite, enargite, and chalcopy-rite, place the timing of these breccias after the pyrite-chalcopyrite ± bornite mineralization, but before the massivepyrite-chalcocite-covellite. Relict A or AB-type quartz veins,deformed and cracked, with chalcocite filling in the cracks(Fig. 6g) are found in the high-grade part of the T orebody,and quartz-covellite-enargite veins with minor anhydrite,enargite fractured and overgrown by covellite similar to thecovellite that constitutes the matrix with residual quartz (Fig.7i) are present at the periphery, suggesting that the massivesulfide replacement overprints all other mineralization stylesand represent the terminal stage of ore formation or the mostdistal parts of telescoped magmatic-hydrothermal system.

Whole-rock geochemistry and alterationmassbalance

Motivation and analytical approach

The purpose of this section is to test whether the texturalevidence for progressive massive sulfide formation by whole-sale replacement of an andesitic protolith is supported bychemical mass balance, to assess the overall volume changeassociated with this process, and to derive bulk fluid-mineralreactions describing the main types of alteration and mineral-ization. Samples from the high-grade core of the T orebodyand from the edges with advanced argillic alteration and a

Fig. 8 Paragenetic sequence of porphyry-epithermal-massive sulfide sys-tem at Bor based on vein cross-cutting relationships and alterationoverprinting patterns (Fig. 5), as well as microscopic evidence (Fig. 7).

Assemblages towards the right are seen to overprint those further left,from column to column and within each column, but local reverse rela-tions indicate that timing might be overlapping on the mine scale

Miner Deposita

minimally altered andesite 900m to the side of it were used forthis comparison (Digital Appendix A).

Quantifying mass changes requires analyses of all majorelements, including Si and Al as well as Fe and S in verydifferent rock types, from the silicate protolith containing mi-nor amount of sulfides to the massive sulfide-dominated sam-ple containing only minor silicates. Such analyses are notobtained by conventional XRF bulk rock analysis nor by com-mercial four-acid digestion used by industry for multi-elementsulfide ores, which in particular fails to provide reliable Sianalyses. Therefore, a modified procedure for the preparationof XRF beads was used to quantify all required elements in aconsistent way (Willis 2010). Approximately 50–70 g of eachsample was crushed in an agate mill. A 0.5 g aliquot of samplepowder was mixed with 1.5 g of LiNO3 and 6.6 g of Li2O4B7in precisely weighed proportions. The mixture was first grad-ually heated from 300 to 800 °C during 1 h, to allow forcomplete oxidation of sulfide minerals (mostly pyrite) by thenitrate to sulfates. In the absence of a strong smell, we assumethat this sulfide oxidation was complete and did not involvesignificant H2S loss (Willis 2010). Subsequently, the oxidizedmixture was molten and fully homogenized at 1000 °C.Quenched glass beads were analyzed for major elements (Si,Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, S, Cu) by X-ray fluores-cence (XRF) using an Axios PANalytical WD-XRF spec-trometer at ETH Zürich and quantified with 34 standard ref-erence materials and an advanced matrix correction schemeimplemented in the PANalytical software (Brouwer 2006).Trace elements were subsequently determined by laser abla-tion – inductively coupled plasma – mass spectrometry (LA-ICP-MS) at ETH Zürich (Switzerland) on small shards ofbroken glass beads, using at least 3 ablation spots of 115 μmdiameter, a repetition rate of 10 Hz, and a laser energy densityof 8–10 J∙cm−2. NIST 610 glass reference material served asan external calibration standard, and the TiO2 content of thesample obtained by XRF was used as internal standard for theLA-ICP-MS traces. LA-ICP-MS intensities were processedusing the MatLab-based SILLS software (Guillonget al. 2008). Concentrations of gold were taken frommine assays for the corresponding 2-m intervals to re-duce the nugget effect.

This combination of XRF and LA-ICP-MS gives compa-rable bulk rock element concentrations even of samples withhighly different matrices (Vuchkova and Jordanov 2000;Günther et al. 2001; Khokhlova et al. 2014; Ling et al.2014). Table 1 (top) lists the analyzed concentrations of majormetals and sulfur (by XRF) as well as trace elements (P, Mnby XRF, others by LA-ICP-MS). Unlike conventional silicaterock analyses, the concentrations of O, H, and the elementtotals cannot be readily measured and calculated, because (a)Fe (II) and Fe(III) are not measured due to pre-oxidation and(b) the weight change on ignition reflects not only H2O loss,but also oxidation of several sulfide minerals; i.e., two effects

that cannot be separated by analytical methods alone. Instead,we first recalculated the analyses from wt% metals and sulfurinto normative molar quantities of a set of plausible mineralcomponents, including simplified mineral endmembersand exchange components describing solid solutions(Thompson 1982).

This norm calculation (reported in Table 1, bottom) servestwo purposes. First, it allows calculation of O and Hwhich arenot available by analysis. We thereby also obtain a roughestimate of the element totals for the complex mixed silicate-sulfide rocks, as reported in the corresponding three lines ofTable 1 (top). These totals are well below the 100 ± 2 wt%normally accepted for conventional bulk silicate rock analy-ses, but they are considered acceptable considering the com-plex rock matrices and the simplification in the choice ofmineral components. For the following derivation of masschanges, we normalized the element concentrations includingthe estimated O and H to 1000 g of rock. Secondly, the min-eral norm defines the amounts of minerals in the least-alteredprotolith, and the amounts of newly formed minerals in eachof the altered rocks. These amounts delimit the quantities ofminerals available as educts and products in bulk alterationreactions to be derived later.

The calculation of approximate modal mineralogy (mineralnorm) from the analyzed element concentrations (in wt% con-centrations) was based on the petrographically observedmajorminerals in each rock. Mathematically, the 10 analyticallymeasured major element concentrations can be uniquelyrecalculated to 10 mineral components by base transforma-tion, if the chosenmineral components fully describe the com-position space and the bulk rock analysis is perfect, resultingin modal abundances that are all positive numbers (Thompson1982). With complex solid solutions, mineral heterogeneities,and uncertainties in rock analysis, it may not be possible toobtain modal numbers that are all positive. To estimate real-istic modal proportions listed in Table 1, we therefore itera-tively modeled combinations of petrographically observedphases using the SOLVER optimization tool in Excel(Lasdon et al. 1978), which allows imposing the non-negativity constraint and setting the deviation tolerance ofidealized mineral quantities from the measured element con-centrations, resulting in a number of plausible modal abun-dances that may be larger than 10. To avoid introducing mul-tiple Fe and Mg endmembers of ferromagnesian silicates (am-phibole, biotite, chlorite), an Mg endmember for each mineralwas used together with the exchange components FeMg−1, tosatisfy mass balance without considering the distribution ofthe two elements among ferromagnesian solid solutions(Heinrich et al. 1995). The minerals that successfully describethe bulk composition of each rock will later be used for for-mulating alteration reactions in terms of the observed protolithand alteration minerals and plausible aqueous componentsthat occur in the mineralizing fluids. For the protolith, we

Miner Deposita

Table1

Com

positebulk

rock

analyses

combining

XRFandLA-ICP-MS,

expressedin

wtu

nitsof

elem

entsa

Whole-rockdataforgeochemicalmassbalancecalculation.SeeDigitalAppendixAforrawdataandcalculation

Rocktype

Protolith

Monom

ictic

breccia

with

phyllic-argillic

alteredclasts

Silicifiedadvanced

argillicbreccia

Advancedargillic

alteratio

nVuggy

residual

quartz

Massive

sulfide

with

residualquartz

Massive

sulfide

Sample

B67-258

BB4–2.5

BB1-b2

BA2–85.5

BB4–55.8

BB3–23

BB4–48

Location

(see

Fig.2)

DrillholeB67,258

mDrillholeBB4,2.5m

DrillholeBB1,5–10

mDrillholeBA2,85.5

mDrillholeBB4,55.8

mDrillholeBB3,23

mDrillholeBB4,48

m

Rockdescription

Ampandbt

replaced

bychlandserin

qz-plma-

trix;am

p20%,pl

30%,

chl15%,bt10%,qz

10%;accessorypy,ccp,

mag

Claststo

2cm

with

ser

(after

plan

dam

p);

matrixpy

+kln+qz;qz

40%,kln30%,py15%,

ser10%;accessory

anh,

bn,ccp,rt

Clastwith

ampreplaced

bypy

;kln+qz

matrix;

cctandpy

veinlets;qz

30%,kln30%,py

25%,

ser10

%;accessorycv

andanh

Pyafteram

pandmag,

matrixwithdsp,

py,

kln,

qz;py

20%,kln

15%,qz

25%,dsp

20%;accessory

ser,

ccp,rt,bn

Relictampreplaced

bypy

,vu

gswithqz,py

,kln;

qz70

%,py

20%;

accessorycv,cct,kln,

anh,rt

Weaklypreserved

textureof

amp

replaced

bypy,m

atrix

ofcctand

cv;py40%,

qz35

%,cct10

%,cv

10%;ac

cessory

rt,

anh,col,kln

Euhedralp

y,fine-grained

cct+

cv,m

icroscopicAu

inclusions;py

40%,cct

35%,cv

10%;accessory

qz,rt,kln

Density,g/cm

32.46

2.98

3.21

3.17

2.65

3.85

4.89

Si28.35

21.44

23.84

21.40

34.07

13.98

0.63

Ti

0.27

0.25

0.19

0.23

0.25

0.07

0.14

Al

8.94

7.37

0.59

6.92

0.38

0.04

0.02

Fe3.53

10.69

15.34

16.98

8.30

19.52

21.50

Mn

0.07

0.01

0.01

0.01

0.01

0.01

0.01

Mg

1.63

0.12

0.17

0.12

0.18

0.18

0.18

Ca

4.23

1.40

0.55

0.12

0.84

0.10

0.12

Na

2.12

0.03

0.07

0.13

0.00

0.00

0.00

K0.84

0.08

0.01

0.08

0.02

0.002

0.002

Cu

0.01

0.39

1.69

0.76

0.03

11.04

30.32

S0.43

12.86

17.85

20.36

10.21

27.58

32.28

Hb

0.09

0.52

0.03

0.39

0.03

0.01

0.01

Oc

45.34

36.72

30.00

35.10

40.06

16.35

1.22

Totald

95.84

91.87

90.34

102.59

94.37

88.87

86.43

P120

323

57124

3924

37

Sc25

8.7

4.5

16.5

3.2

1.31

1.70

V226

129

23214

2288

58

Co

13.1

16.2

12.3

3415.3

3.4

4.5

Ni

395

852

120.0

6.2

10.7

5.2

17.4

Zn

435.9

2019.2

5.2

15.2

9.7

Ge

183.9

6.9

204.8

14.8

11.1

As

12.3

279

1210

393.5

271

134

Sr887

1330

769

629

334

6.6

15.7

Miner Deposita

Tab

le1

(contin

ued)

Whole-rockdataforgeochemicalmassbalancecalculation.SeeDigitalAppendixAforrawdataandcalculation

Rocktype

Protolith

Monom

ictic

breccia

with

phyllic-argillic

alteredclasts

Silicifiedadvanced

argillicbreccia

Advancedargillic

alteratio

nVuggy

residual

quartz

Massive

sulfide

with

residualquartz

Massive

sulfide

Y12.6

1.82

2.4

3.3

1.03

0.14

0.28

Zr

4439.8

3740

4024

26

Nb

3.0

2.5

2.0

2.3

2.5

0.65

1.22

Mo

5.1

2.8

3.1

1.54

2.1

2.8

16.4

Ag

dl1.01

4.7

0.35

1.23

5.7

24

Sn2.1

9.0

4714.3

4.2

171

96

Ba

329

584

7250

4910

1670

3.4

261

La

4.4

5.8

3.7

5.4

4.1

1.60

1.36

Ce

10.4

7.7

3.9

12.5

6.8

0.68

0.38

Pr1.32

0.93

0.44

1.54

0.84

0.08

0.03

Nd

7.1

3.7

1.91

7.9

3.1

0.38

0.21

Gd

1.53

0.81

3.8

1.04

1.03

0.19

0.24

Dy

1.57

0.27

0.16

0.30

0.04

0.04

0.01

Yb

0.95

0.31

0.26

0.36

0.09

0.06

0.07

Lu

0.07

0.06

0.05

dldl

0.02

dl

Hf

0.95

1.08

1.00

0.83

1.30

0.64

0.87

Aue

0.10

1.00

1.20

3.3

7.8

9.3

25

Tl

0.05

0.06

0.43

1.09

dl0.11

dl

Pb1.45

5979

2614.7

17.5

6.6

Th

1.28

1.50

0.76

1.07

1.19

0.16

0.21

U0.36

0.41

0.23

0.28

0.32

0.09

0.06

Normativecompositio

nf:m

oles

ofmineralsper1kg

ofrock

Quartz

4.4

5.2

9.4

6.1

12.6

5.5

0.15

Albite

0.95

0.01

0.03

0.06

Anorthite

0.56

Tremolite

0.25

Phlogopite

0.21

Magnetite

0.003

Hem

atite

0.09

0.01

Rutile

0.06

0.06

0.04

0.05

0.06

0.02

0.03

Clin

ochlore

0.03

0.02

0.01

0.02

Talc

0.03

0.03

Miner Deposita

Tab

le1

(contin

ued)

Whole-rockdataforgeochemicalmassbalancecalculation.SeeDigitalAppendixAforrawdataandcalculation

Rocktype

Protolith

Monom

ictic

breccia

with

phyllic-argillic

alteredclasts

Silicifiedadvanced

argillicbreccia

Advancedargillic

alteratio

nVuggy

residual

quartz

Massive

sulfide

with

residualquartz

Massive

sulfide

Muscovite

0.02

0.002

0.02

0.01

0.001

0.001

Anhydrite

0.38

0.15

0.03

0.22

0.03

0.04

Kaolin

ite1.36

0.01

0.62

0.03

0.01

0.002

Diaspore

0.13

1.09

0.04

0.001

0.001

Pyrite

0.07

1.96

2.87

2.98

1.59

3.93

4.37

Chalcopyrite

0.002

0.04

0.02

Bornite

0.01

0.02

Covellite

0.29

0.01

1.61

0.29

Chalcocite

0.001

0.17

2.63

Fe-M

g-exchange

0.59

0.09

0.01

Tscherm

akexchange

0.59

aMajor

elem

ents(m

etals,S)in

weightp

ercent

measuredby

XRF,trace

elem

entsin

ppm

measuredby

LA-ICP-M

Sof

tetraboratepills,exceptA

ufrom

company

assays

bHestim

ated

from

thecontento

fOH-bearing

mineralsbasedon

thenorm

calculationin

thelower

partof

thetable(A

ppendixA)

cOestim

ated

from

thecontento

fsilicateandoxidemineralsbasedon

thenorm

calculationin

thelower

partof

thetable(A

ppendixA)

dElementtotalsincludingOandHobtained

bynorm

calculation

eAucontentfrom

company

assaydata,R

TBBor,averagedover

2m

ofcore

neighboringouranalyzed

smallersample

fEstimated

mineralnorm

calculated

from

theanalyzed

metalandSconcentrations,based

onobserved

mineralogyandoptim

izationprocedureexplainedinthetextandtheDigitalR

epository

fileAppendixA

Mineralform

ulae:albite

=NaA

lSi 3O8,anhydrite=CaSO4,anorthite=CaA

l 2Si

2O8,b

ornite=Cu 5FeS

4,chalcocite

=Cu 2S,

chalcopyrite=CuF

eS2,clin

ochlore=Mg 5Al(Si 3Al)O10(O

H) 8,covellite=CuS

,diaspore=AlO(O

H),hematite

=Fe 2O3,kaolinite

=Al 2Si 2O5(O

H) 4,magnetite=Fe 3O4,muscovite=KAl 2(A

lSi 3O10)(OH) 2,phlogopite=KMg 3AlSi 3O10(F,OH) 2,pyrite=FeS

2,quartz=SiO2,rutile=

TiO

2,tremolite

=Ca 2Mg 5Si

8O22(O

H) 2,talc=Mg 3Si

4O10(O

H) 2.F

erromagnesian

minerals(amphibole,biotite,chlorite)areapproxim

ated

byMgendm

embers,and

Fe,S

i,andAld

istributed

overseveral

solid

solutio

nphases

areexpressedas

exchange

components,FeMg −

1andAl 2Si −1Mg −

1(Tscherm

akexchange)

Miner Deposita

Miner Deposita

choose to express the norm in terms of simplified igneousminerals, although partial alteration of amphibole to chloriteand plagioclase to kaolinite and anhydrite is observedpetrographically.

Gains, losses, and volume change

We chose amodified version of the procedure of Grant (2005) tocompare each altered sample on the vertical scale of the isocondiagrams in Fig. 9 with the same protolith plotted on the hori-zontal. This is the basis for interpreting the relative mobility ofelements during the different types of alteration, including mas-sive sulfide formation. To avoid visual bias, element concentra-tions were normalized to the sum of squares for each sample pairof altered rock and protolith (Humphris et al. 1998).Consequently, when plotting each normalized concentration inthe altered rock against the normalized concentrations of theelement in the protolith, the resulting point lies on a circle withradius 1 (Fig. 9). Elements that group closely together along thiscircle have maintained the same element ratio, and this can betaken as an indication that this group of elements was not signif-icantly mobile during alteration (Grant 2005). Only a few ele-ments occur close together along the circle for all rock pairs, butthese include typically immobile Ti, Zr, Hf, U, and Th. Rutile ispresent in every altered sample, its textural character does notvary much from sample to sample; even in the massive sulfideore with only a small amount of residual quartz, rutile is stillvisible as distinct grain aggregates. Therefore, in situ precipita-tion of rutile aftermagnetite and amphibole phenocrysts supportsthe assumption that Ti is our best approximation for a conservedcomponent despite the widespread evidence of its mobility in thegeneral case of high-sulfidation epithermal deposits.Whenever aTi-bearing phase was subject to alteration and leaching, the lib-erated Ti precipitated as rutile and was transported at a scale notmuch greater than that of the original mineral grains (few mm orless). Zircon is the only phase that incorporates Zr and Hf in

significant quantities and is also robust to alteration, but theabundance of zircon is much less than the abundance of Ti andis probably subject to greater analytical uncertainty and randomscatter. We therefore selected Ti as a reference element for thecalculation of gains and losses, as indicated by the reference linein the isocon plots of Fig. 9.

The two breccia types from the bottom of the T orebody(Fig. 9a, b) indicate close agreement between the Ti conser-vation and isovolumetric reference lines. The assumption ofTi immobility agrees with other candidates for immobile be-havior (Hf, Zr, Th, U); i.e., the ratios among these elementsdid not change so that the assumption that they were all im-mobile is plausible. The Ti isocon line for both breccias closeto the line of isovolumetric replacement is consistent with thehigh degree of textural preservation. Silicon is mobilized to asmall degree only: slightly lost from the monomictic breccia(Fig. 9a) or slightly gained by the silicified advanced argillicbreccia (Fig. 9b). Aluminum is approximately conserved inthe monomictic breccia but severely lost from the silicifiedadvanced argillic breccia. Major alkali and earth alkali ele-ments were significantly depleted during alteration of bothbreccias, whereas Cu, S, Au, and a number of minor ore ele-ments (Pb, As, Sn, and also Fe) were consistently gained asprecipitating sulfide minerals, consistent with abundant dis-seminated pyrite and the economic ore grade of these rocks(0.6 and 1.7% Cu, ~ 1 ppm Au).

Advanced argillic alteration sample and vuggy residualquartz (Fig. 9c, d) are also characterized by close correspon-dence between the location of the Ti reference line, comparableratios to weakly mobile Th, U, Zr, and Hf, as well as the con-stant volume line. In the vuggy residual quartz sample (Fig. 9d),only gold and pyrite (Fe, S) are visibly enriched, while all otherore-forming metals are either insignificantly enriched (notablyCu) or even depleted (As). Silicon plots close to the isocon linein the advanced argillic alteration sample (Fig. 9c) but is signif-icantly enriched in the vuggy residual quartz sample (Fig. 9d).Aluminum is conserved in the advanced argillic alteration sam-ple but is lost, together with all other rock-forming elementsother than Si, in the vuggy residual quartz sample.

Massive sulfide samples (Fig. 9e, f) show the greatest de-viation of the constant volume line from the Ti reference line(Fig. 9e), as well as the largest scatter of other potentiallyimmobile Th, U, Zr, and Hf (Fig. 9e, f). Nevertheless, Ticonservation is the most plausible reference considering thetextural preservation of rutile clusters even in these extremelyaltered rocks (Fig. 7f and g). Si and Zn, as well as Zr, plotbetween the Ti reference line and the constant volume line inthe massive sulfide sample with a significant amount of resid-ual quartz (Fig. 9e). All ore-forming elements are enriched,which is illustrated by large amount of copper sulfides, dom-inantly covellite for the massive sulfide sample with residualquartz, and chalcocite in the case of truly massive sulfidesample, consistent with extremely high copper and gold

Fig. 9 a–f Re-scaled concentrations of major and trace elements, empha-sizing main ore elements (red symbols), minor chalcophile andsiderophile elements (blue), main rock-forming and lithophile elements(green), and potentially immobile trace elements (black). The black line isthe reference line assuming that Ti remained conserved (Ti isocon line),the red line based on measured densities represents the reference if alter-ation would involve zero volume change. With Ti as referenceassumption, elements above the black line experienced gains, el-ements below the line experienced loss during alteration.Rescaling is done by normalizing to the sum of squares of the

measured elements: X s ¼ X2a

X 2aþY 2a; Y s ¼Y 2a

X 2aþY 2a; where Xa and Xs refer toanalyzed and re-scaled content of an element for the protolith, and Ya andYs refer to analyzed and re-scaled element content for the altered sample.Each diagram also shows the reciprocal ratio of measured rock densities(red lines) to illustrate the alternative assumption of isovolumetric re-placement (Gresens 1967). Assuming that Ti remained conserved, redlines above the Ti isocon (black line) indicate net volume gain, thosebelow indicate net volume loss during alteration

Miner Deposita

grades (11 and 30% Cu, 9 and 25 ppm Au). Most rock-forming elements are extremely depleted (Fig. 9e), in line withthe complete destruction of rock-forming minerals. Massivesulfide with only small amount of residual quartz displays acoincidence between the constant volume line and the Ti ref-erence line; Zr plots close to the isocon, but Hf, Th, and Udeviate from both reference lines (Fig. 9f). Main rock-formingelements are depleted in the same manner as for the massivesulfide with residual quartz, except that Si is completely lost inthe truly massive sulfide sample together with other rock-forming elements (Fig. 9f).

Assuming the conservation of Ti, gains and losses for eachelement during each type of alteration were calculated accord-ing to the formula (Grant 2005):

ΔC ¼ CPimmobile−CAimmobile� �

⋅CA−CP ð1Þ

where CP is the concentration of element in the least-alteredprotolith, andCA is the concentration in the altered sample. Allaltered and mineralized samples are compared to the freshandesite protolith, on the initial assumption that each is de-rived directly from the same andesite. The resulting gains orlosses (positive or negative ΔC) of each mobile element areinitially expressed in weight percent of major or in ppm oftrace elements (Table 1).

Bulk fluid-rock reactions

Mass gains and losses in units of elements (or oxides such asNa2O) are far from a geologically realistic description of fluid-rock reaction, which in reality involves mineral phases in therock and aqueous components in the fluid. In this section, wetherefore recalculate the mass balance results of Table 1 intomore plausible bulk fluid-mineral reactions, using the min-erals that make up the norms of the protolith and the variablyaltered rocks (Table 1) and a set of simplified aqueous com-ponents (cf. Heinrich et al. 1995; Ulrich and Heinrich 2002;Halley 2020). All reactions are expressed in mole units andscaled to changes experienced by 1 kg of protolith.

The stoichiometric coefficient ΔΝminR of each mineral

component (subscript min) in the reaction R is the differencebetween the molar abundance of each mineral making up thealtered rock (A), minus the corresponding moles making upthe protolith (P), whereby the altered rock is scaled to preservethe total quantity of Ti, assumed to be conserved:

ΔNRmin ¼CATiCPTi

⋅NAmin

� �−NPmin ð2Þ

Thus, minerals only existing in the protolith are completelyconsumed, and their abundance in the protolith equals theirstoichiometric coefficient on the left-hand side (educts) of thereaction. Conversely, minerals only present in the altered rockappear on the right-hand side (products) of the reaction, with

stoichiometric coefficients corresponding to their abundance inthe new rock, scaled to constant Ti. The digital Appendix Aexplains the details of the calculation and includes a forwardcalculation of gains and losses in weight% of elements, to checkthe accuracy in terms of the analyzed concentrations (Eq. 2).

The aqueous reaction partners represent gains by the reactingfluid compensating the element losses experienced by thereacting rock. Conversely, element gains by the rock must beprovided by the fluid, which therefore experiences a correspond-ing loss of elements. The stoichiometric coefficients of the cho-sen aqueous species are calculated by converting the gains andlosses in grams of analyzed elements per 1000-g protolith (Eq. 1)to the corresponding molar quantities, using the same matrixinversion as used in the norm calculations ofmineral components(see Appendix A). As aqueous solutions containing ionic com-ponents need to be charge-balanced, an additional element mustbe included to constrain this base transformation, i.e., electrons.Alternatively, we here use neutral species including chloride saltsfor most metals that are chloride-complexed in saline solutions,thus requiring Cl as additional, conserved component. We intro-duce HCl as the charge-compensating aqueous component,which provides a useful measure of net acid gain or loss attend-ing alteration. This acid balance includes the hydrolysis of anhy-drous minerals such as K-feldspar to OH-bearing minerals suchas muscovite and kaolinite, as well as the net acidity produced byany metal precipitation reaction (cf. Hemley and Hunt 1992;Reed et al. 2013). Sulfate and sulfide are the likely dominant pairof oxidized and reduced species in the fluid and we therefore useH2SO4 and H2S as aqueous components to account for redoxbalance. Total sulfate and sulfide in the fluid thus express the netredox changes among Fe-silicates of variable Fe valency and ofsulfates and sulfides in the rock (Drummond and Ohmoto 1985;Giggenbach 1992; Ohmoto and Goldhaber 1997).

To obtain stoichiometrically balanced reactions, we calcu-late gains and losses based on the idealized composition thatcorrespond exactly to the mineral norm. Appendix A containsan extra sheet for the sample pair BB4-2.5/B67-258 to dem-onstrate that deviations never exceed 5% of the analyzed con-centration of each element. Nevertheless, some reactions ap-pear slightly unbalanced on their last digit of stoichiometriccoefficients, due to rounding (correct values see Appendix A).

For simplicity, minor Mn and P were omitted. Mn is small-er than the likely analytical uncertainty of major element con-centrations, but is consistently lost during all alteration types.Phosphorus is contained in apatite, which occurs in the heavymineral fraction but was not seen in thin section; it is lost frommost alteration types, except for the monomictic breccia withphyllic-argillic altered clasts (BB4-2.5), where gain of P po-tentially indicates the presence of alunite or some undetectedaluminum-phosphate-sulfate minerals. In the following, wewill present each bulk reaction obtained in this manner andassess its geological significance with regard to major reactionpartners.

Miner Deposita

Monomictic breccia with phyllic-argillic altered clasts (BB4-2.5) This breccia from the edge of the T orebody was formedby the destruction of magmatic amphibole + biotite + plagioclasewith intermediate Ab/An (andesine-labradorite) to a slightly

smaller volume of denser rock in which quartz + pyrite + kao-linite formed together with anhydrite, chlorite, and minor chal-copyrite. This alteration involved a slight removal of Si and Albut required a major introduction of FeCl2 (Fig. 9a). It also

Table 2 Gains and losses of mineralized samples in comparison to weakly altered andesite protolith

Rock type Monomictic brecciawith phyllic-argillicaltered clasts

Silicified advancedargillic breccia

Advanced argillicalteration

Vuggy residualquartz

Massive sulfidewith residual quartz

Massive sulfide

Sample pair BB4-2.5 / B67-258 BB1-b2 / B67-258 BA2-85.5/B67-258 BB4-55.8/B67-258 BB3-23/B67-258 BB4-48/B67-258

Si − 60 56 − 23 78 276 − 280Ti 0 0 0 0 0 0

Al − 14.7 − 85.0 − 9.2 − 89.3 − 91.4 − 92.9Fe 82 183 175 55 756 390

Mg − 15.9 − 14.7 − 15.7 − 15.2 − 10.2 − 13.9Ca − 27 − 34 − 41 − 33 − 38 − 40Na − 22 − 21 − 20 − 22 − 22 − 22K − 7.5 − 8.2 − 7.4 − 8.2 − 8.3 − 8.3Cu 4.2 24 9.1 0.22 449 614

S 140 252 242 109 1117 649

H 4.9 − 0.5 3.8 − 0.6 − 0.6 − 0.8O −61 − 43 − 47 − 27 192 − 448P 227 − 59.0 13.7 − 78 − 94 − 80Sc − 15.3 − 19.8 − 6.9 −21 − 23 − 23V − 88 − 201 3.8 −202 − 132 − 164Co 4.3 0.15 24 3.4 − 9.4 − 8.3Ni 521 − 266 − 388 − 383 − 389 − 376Zn − 37 − 22 − 23 − 38 − 27 − 33Ge − 13.7 − 10.4 3.8 − 12.7 − 1.9 − 6.0As 288 1290 30 − 8.6 279 132Sr 543 − 60 − 211 − 530 − 880 − 870Y − 10.6 − 9.9 − 9.0 − 11.4 − 12.4 − 12.3Zr − 1.42 − 4.8 − 0.92 − 1.58 − 18.6 − 16.2Nb − 0.27 − 0.79 − 0.44 − 0.30 − 2.3 − 1.65Mo − 2.0 − 1.72 − 3.4 − 2.8 − 2.1 12.6Sn 7.6 49 13.3 2.5 181 101

Ba 300 7460 4950 1470 − 326 − 49La 1.83 − 0.43 1.39 0.002 − 2.7 − 3.0Ce − 2.1 − 6.2 3.1 − 3.1 − 9.6 − 10.0Pr − 0.32 − 0.84 0.33 − 0.41 − 1.23 − 1.28Nd − 3.1 − 5.0 1.42 − 3.8 − 6.7 − 6.8Gd − 0.66 2.5 − 0.41 − 0.42 − 1.33 − 1.27Yb − 0.62 − 0.67 − 0.56 − 0.86 − 0.88 − 0.87Hf 0.21 0.13 − 0.06 0.45 − 0.26 − 0.01Au 0.98 1.19 3.4 8.3 9.9 27

Pb 62 84 26 14.3 17.4 5.7

Th 0.33 − 0.46 − 0.13 0.01 − 1.10 − 1.05U 0.08 − 0.12 − 0.06 − 0.02 − 0.26 − 0.30

Gains (positive values) and losses (negative) of major elements and trace elements in comparison to the protolith sample B67-258

Major elements in grams, trace elements in μg per 1 kg of protolith

Miner Deposita

required input of oxidized, as well as reduced sulfur and acidity,whereas most of the alkali and earth alkali metals were lost aschloride salts (Fig. 9a), apart from the small fraction of Ca incor-porated into anhydrite. The net introduction of HCl, but stoichio-metric loss of H2O, means that the increase in contained waterbound in kaolinite was mainly due to hydrolysis of less hydrousigneous minerals (cation exchange), requiring an acid fluid thatalso is capable of net Al removal. Even though the rock experi-enced the formation of 1.4 mol of quartz, the production ofSiO2,aq was somewhat larger at 2.1 mol, as observed by the netSi loss during destruction of the magmatic silicate minerals(Table 2, Fig. 9a). The rock changed from almost barren to loweconomic ore grade with net addition of Cu to form chalcopyrite,consistent with petrography.

1000 g or 407 cm3 of andesiteprotolith with ρ = 2.46 g/cm3

1019 g or 342 cm3 monomicticbreccia with phyllic-argillic alteredclasts, BB4–2.5 withρ = 2.98 g/cm3

0.249 Trem + 0.938 Ab +0.558 An + 0.214 Phl +0.003 Mt + 0.496 FeMg−1 +0.588 Al2Si−1Mg−1 +1.471 FeCl2 + 0.066 CuCl +3.475 H2S + 0.879 H2SO4 +0.761 HCl

→ 0.03 Clnchl + 0.392 Anh +1.386 Kln + 1.932 Py +0.022 Mus + 0.929 Qtz +0.039 Ccp + 0.005 Bn +2.122 SiO2,aq + 0.545 Al(OH)3 +0.654 MgCl2 +0.665 CaCl2 + 0.938 NaCl +0.192 KCl + 1.467 H2O

Ni and Co are enriched in the mildly altered rocks, bytransfer from igneous minerals to relatively minor pyrite underacid-neutralizing conditions (Fig. 9a); Mo and Zn experienceda net loss. Loss of Zn in this sample and in all other samplesfrom the T orebody is probably due to elevated level of Zn thatis explained by weak chlorite alteration of the protolith;overprinting of chlorite by other alteration types would resultin the destruction of chlorite and Zn loss. Minor and traceelement composition changes include the addition of P, Ba,and Sr (Table 2, Fig. 9a), which may be incorporated intoanhydrite. Many lithophile trace elements including mostREE, experienced net loss, probably by the destruction ofigneous minerals, especially magnetite, without the formationof new host accessories. Gain of P and simultaneous loss ofmost REE can be explained by the formation of minoraluminum-phosphate-sulfate minerals not identified in thesample but common in Bor (Janković et al. 2002).

Silicified advanced argillic breccia (BB1-b2) This breccia isformed by converting the protolith into a denser rock, whichoccupies almost the same volume due to net mass gain. Oreformation is expressed by the production of larger amount ofchalcopyrite and pyrite compared to the monomictic brecciawith phyllic-argillic altered clasts BB4-2.5 and is accompa-nied by smaller amounts of anhydrite formed and more sig-nificant loss of Al. This alteration requires further addition ofoxidized and reduced sulfur as well as FeCl2. The Cu-Fe-

sulfide addition and net oxidation of the rock by sulfuric acidare accompanied by liberation of additional HCl into the fluid.

1000 g or 407 cm3 of andesiteprotolith with ρ = 2.46 g/cm3

1304 g or 406 cm3 silicifiedadvanced argillic breccia, BB1-b2with ρ = 3.21 g/cm3

0.249 Trem + 0.908 Ab +0.558 An + 0.214 Phl +0.003 Mt + 0.002 Ccp +0.578 FeMg−1 +0.588 Al2Si−1Mg−1 +1.998 SiO2, aq +3.27 FeCl2 + 0.381 CuCl +6.663 H2S + 1.18 H2SO4

→ 0.024 Clnchl + 0.197 Anh +0.015 Kln + 0.174 Dia +0.003 Mus + 3.635 Py +7.776 Qtz + 0.112 Hem +0.38 Cv + 0.001 Cc +3.152 Al(OH)3+ 0.604 MgCl2 +0.86 CaCl2 + 0.908 NaCl +0.211 KCl+1.926 H2O+2.875 HCl

As, Pb, and Sn are significantly enriched and areprobably incorporated into pyrite and covellite; Coshows weak enrichment, and Mo is lost in this sample.Loss of P accompanied by loss of Ca, as well as mostREE, can indicate destruction of igneous apatite, andgains of minor chalcophile elements are reflected by amore pronounced sulfide enrichment of this brecciacompared to the BB4-2.5 sample (Fig. 9b).

Advanced argillic alteration (BA2-85.5) This sample is formedby almost complete replacement of former hornblende and biotitephenocrysts by quartz, pyrite, sericite, and kaolinite + diaspore,with a smaller amount of anhydrite compared to what is foundin the breccias, as well as the complete destruction of magnetite.The resulting volume differs from the protolith by 3%, due to theeffect of counterbalancing the mass gains expressed in formationof denser minerals like Cu sulfides and diaspore. This alterationrequires the addition of both oxidized and reduced sulfur, but seeslosses of H2O andHCl due to the drastic amounts of rock-formingelements that need to be removed in the form of chloride com-plexes, which prevails compared to the addition of ore-formingelement chlorides.

1000 g or 407 cm3 of andesiteprotolith with ρ = 2.46 g/cm3

1262 g or 399 cm3 advancedargillic alteration, BA2–85.5 withρ = 3.17 g/cm3

0.249 Trem + 0.881 Ab +0.558 An + 0.214 Phl +0.003 Mt + 0.59 FeMg−1 +0.588 Al2Si−1Mg−1 +3.141 FeCl2 + 0.142 CuCl +6.598 H2S + 0.964 H2SO4

→ 0.013 Clnchl + 0.036 Anh +0.785 Kln + 1.374 Dia +0.025 Mus + 3.689 Py +3.317 Qtz + 0.028 Ccp +0.023 Bn + 0.807 SiO2,aq +0.342 Al(OH)3 + 0.648 MgCl2 +1.021 CaCl2 + 0.881 NaCl +0.189 KCl + 4.171 H2O +2.017 HCl

All minor chalcophile elements except Zn show consider-able enrichment; Mo is moderately depleted, and Ni is lostcompletely. Minor gain of Co (Table 2) can reflect its incor-poration into pyrite that developed after magnetite (Fig. 9c), or

Miner Deposita

into chalcopyrite. Moderate enrichment of P is similar to theenrichment of Ce, La, Pr, and Nd.

Vuggy residual quartz (BB4-55.8) In this sample, the effect ofleaching of original plagioclase and hornblende phenocrystsand formation of vugs is partly counterbalanced by the in-creased amount of pyrite, illustrating the close-to-isovolumetric replacement of the andesite protolith. Basedon the following reaction of converting the protolith to vuggyresidual quartz, the required addition of SiO2 is not large:conversion of the matrix of the protolith to quartz serves asthe most important source of silica, which is accompanied bycomplete destruction of all rock-forming minerals and remov-al of alkalis, Ca and Al into solution. The addition of reducedsulfur is necessary, as well as acidity, to leach all the rock-forming oxides and leave the carcass of residual quartz withpyrite and kaolinite. Net addition of water is insignificant,which shows that for this pair of samples all the hydroxidesthat were contained in the rock-forming minerals could beincorporated into kaolinite or else required for the removalof Al.

1000 g or 407 cm3 of andesiteprotolith with ρ = 2.46 g/cm3

1047 g or 395 cm3 Vuggyresidual quartz, BB4-55.8 withρ = 2.65 g/cm3

0.249 Trem + 0.951 Ab +0.558 An + 0.214 Phl +0.003 Mt + 0.002 Ccp +0.59 FeMg−1 +0.588 Al2Si−1 Mg−1 +2.783 SiO2,aq + 0.985 FeCl2 +0.004 CuCl + 2.779 H2S +0.201 H2O + 0.622 H2SO4 +2.101 HCl

→ 0.017 Clnchl + 0.225 Anh +0.029 Kln + 0.04 Dia +0.005 Mus + 1.587 Py +8.677 Qtz + 0.006 Cv +3.308 Al(OH)3+ 0.626MgCl2 +0.832 CaCl2 + 0.951 NaCl +0.209 KCl

Virtually all minor chalcophile and siderophile elementsare depleted, with notable exceptions of Sn, Pb, and Co (Fig.9d). All trace elements are lost, reflecting the leaching ofpyrite-enriched rock by highly reactive, corrosive fluid thatleft nothing behind but vuggy residual quartz with significantamount of pyrite and minor amount of copper sulfides.

Massive sulfide with residual quartz (BB3-23) This high-grademassive sulfide sample is formed by bringing most of therock-forming elements except Si into solution, except for thesmall amount taken up by talc and muscovite. Talc is notobserved petrographically, but is arbitrarily introduced as apossible mineral phase that could incorporate the minoramount of analyzed Mg. Added sulfur is predominantly inthe reduced form, and both HCl and H2O losses are requiredto balance the effect of addition of ore metals as chloridecomplexes and to bring Al into solution. The net gain of Siin this sample is higher than in the vuggy residual quartzsample, suggesting net silicification in addition to filling ofthe vuggy quartz carcass with Cu and Fe sulfides. However,

this sample displays maximum deviation of the Ti conserva-tion line from the constant volume line, which is closer to Zr(Fig. 9e), suggesting that Ti is not entirely conserved on alocal scale.

1000 g or 407 cm3 of andesiteprotolith with ρ = 2.46 g/cm3

3557 g or 925 cm3 Massivesulfide with residual quartz,BB3-23 with ρ = 3.85 g/cm3

0.249 Trem + 0.951 Ab +0.558 An + 0.214 Phl +0.003 Mt + 0.002 Ccp +0.59 FeMg−1 +0.588 Al2Si−1Mg−1 +9.815 SiO2,aq +13.543 FeCl2 + 7.062 CuCl +30.466 H2S + 4.363 H2SO4

→ 0.1 Anh + 0.029 Kln +0.003 Dia + 0.003 Mus +14.144 Py + 15.379 Qtz +0.62 Cc + 5.824 Cv +0.098 Tlc + 3.389 Al(OH)3 +0.418 MgCl2 + 0.957 CaCl2 +0.951 NaCl + 0.211 KCl +14.933 H2O + 30.235 HCl

Minor chalcophile elements are all enriched except Zn andcan be incorporated into pyrite and covellite (Table 1); Sn isadded as a trace component to the massive sulfides, possiblyas accessory sulfide such as Sn-bearing colusite (Fig. 9e).Minor siderophile elements are either lost like Ni, displaynear-conservative behavior like Co, or insignificantlyenriched (Mo). All trace elements except La are depleted.

Massive sulfide (BB4-48) This high-grade sample with a smallamount of residual quartz represents a truly massive sulfidefrom the high-grade (7% Cu average) zone of the T orebody.All rock-forming elements including silica are brought intosolution and removed; addition of large amount of dominantlyreduced sulfur is needed to provide for precipitation of pyriteand chalcocite. This sample is characterized by a close coin-cidence of the isocon line with the constant volume line (Fig.9f), which in fact suggests a small volume shrinkage (− 12%)that accompanies conversion of the protolith to the massivesulfide. The measured density is lower than the density calcu-lated from mineral proportions, which could be due to thepresence of pores and cracks in pyrite that constitute around5% of the grain area or the fact that all chalcocite group min-erals are considered chalcocite in the calculation with an av-erage density of 5.65, while the actual density of digenite canbe lower.

1000 g or 407 cm3 of andesiteprotolith with ρ = 2.46 g/cm3

1723 g or 342 cm3 massive sulfide,BB4-48 with ρ = 4.89 g/cm3

0.249 Trem + 0.951 Ab +0.558 An + 0.214 Phl +4.155 Qtz + 0.003 Mt +0.002 Ccp + 0.59 FeMg−1 +0.588 Al2Si−1Mg−1 +6.991 FeCl2 + 9.658 CuCl +18.217 H2S + 2.016 H2SO4

→ 0.062 Anh+ 0.003 Dia +0.003 Kln + 0.001 Mus +7.548 Py + 4.58 Cc +0.5 Cv+ 0.022 Hem +0.047 Tlc + 9.978 SiO2,aq +3.445 Al(OH)3 + 0.572 MgCl2 +0.995 CaCl2 + 0.951 NaCl +0.213 KCl + 5.803 H2O+19.343 HCl

Miner Deposita

Like in the massive sulfide with residual quartz, all minorchalcophile elements are enriched except Zn (Table 2, Fig. 9f).Minor siderophile elements, Ni and Co, are depleted. Loss oftrace elements in the massive sulfide is greater for almost allelements in comparison to any other sample from the Torebody, reflecting complete replacement of the protolith bythe massive sulfide ore.

The overall mass transfer and volume change involved inthe process that generated the entire massive sulfide orebodycan only be discussed subjectively, because we cannot mea-sure the volume fractions of each rock type in three dimen-sions from the incompletely mapped two-dimensional cross-section of the T orebody and the data from our small samples(Fig. 2b). Breccias are restricted to the margins of the Torebody and probably constitute a thin outer shell includingan even smaller fraction of locally silicified breccia. Likewise,the vuggy residual quartz zone overlying the high-grade coretakes up a small fraction of the volume compared with that ofthe massive sulfides, the red region in Fig. 2b. Despite brecciatextures indicating interstitial infilling by hydrothermal min-erals, the chemical mass balance of all these rocks (as well asthe advanced argillic alteration) indicates minor net volumereduction relative to the protolith andesite (between − 1 and −15%), which means that alteration led to the creation of netopen space. The massive sulfides indicate net volume expan-sion in the covellite-dominated sample with relict quartz (Fig.2b), but we infer from the texture of these quartz relics (Fig.7i) that this extra space creation may have been the result ofearlier quartz stockwork veining associated with porphyry-style alteration. The true massive sulfide sample shows a netvolume reduction of 12% and is more representative for theoverall process of massive sulfide replacement, as this oretype is prevalent in the T orebody, as well as in historic sam-ples from Tilva Ros. These observations suggest that the mas-sive sulfide orebody did not form by precipitation into extraspace (e.g., opening of large extensional jogs) but ratherinvolved minor volume reduction by chemical dissolu-tion of andesite.

Discussion and conclusion

This study quantifies the enrichment processes from weaklyaltered andesite protolith to extremely Cu-Au rich massive sul-fide ore. Based on the pieces of evidence such as the presenceof the relict quartz A or AB-type veins in the central part of themassive sulfide orebody and the absence of anhydrite-sulfideveins inside the massive sulfide, as well as similarity of inclu-sions in pyrite from epithermal veins and massive sulfideorebody, we conclude that massive sulfide T orebody presentsthe final mineralization stage of a single telescoped magmatic-hydrothermal system at a deep level, superimposed over por-phyry mineralization style. Mass balance calculation based on

bulk rock analyses, recast into idealized fluid-rock reactionsusing observed minerals and plausible aqueous reaction part-ners added and removed by hydrothermal fluid, indicates thatthe process of formation of almost all mineralization typeswithin the T orebody can be explained by near-isovolumetricreplacement of a common protolith rather than large-scaleopen-space infilling. Although breccias usually are poor candi-dates for demonstrating the direct link to the protolith, we ob-serve clasts of the same lithology with different degree of alter-ation, as well as clasts of veins similar to those observed in thehost andesite, and quite consistent behavior of several typicallyimmobile elements, hence confirming that even the brecciascan be derived from the same single andesite protolith. Theonly mineralization type that cannot be confidently explainedby isovolumetric replacement is the massive sulfide with resid-ual quartz, where either the assumption of Ti conservation isinvalid or open-space infill is more important than replacement,but this may have happened at an earlier stage, most plausiblyintense porphyry-style quartz veining. Considering that themost extreme alteration, pure massive sulfide, as wellas all milder types of alteration satisfies the parametersconsistent with replacement with a small but significantdegree of volume loss, the overall process probably in-cluded minor net volume loss.

Removal of alkali and alkaline-earth elements in high-sulfidation epithermal systems results from the acidity gener-ated by disproportionation of SO2 (Stoffregen 1987; Einaudiet al. 2003), which normally leaves a residual carcass of vuggyquartz. What presents a challenge for the formation of themassive orebodies at Bor is the unusual wholesale removalof quartz during replacive mineralization (Fournier et al. 1982;Akinfiev and Diamond 2009; Monecke et al. 2018). In por-phyry stockwork veins, small-scale but widespread quartz dis-solution can be facilitated by contraction of a cooling single-phase magmatic fluid leading to synchronous quartz dissolu-tion and sulfide precipitation (Landtwing et al. 2010; Feketeet al. 2016). An alternative possibility for large-scale quartzdissolution could be provided by hot magmatic vaporreaching a seawater or meteoric water horizon, therebyheating up a large amount of non-magmatic water. Removalof silica and re-precipitation of quartz in the silicified ad-vanced argillic breccia at the flanks of the massive sulfideorebody supports the hypothesis of highly reactive, corrosivefluid that could turn the protolith into the massive sulfide.Breccias at the bottom of the T orebody could representchannelways for the fluid that formed the massive sulfideswith rich covellite and chalcocite mineralization. The smallbut notably negative volume change of the dominant massivesulfide in the T orebody implies that, after initial brecciationallowing fluid access, the andesite replacement was a self-perpetuating process: the net creation of minor open spacewould have increased rock permeability, thereby contributingto even stronger focusing of additional fluids into the same

Miner Deposita

area. This interpre